1. Field of the Invention
The present invention relates to a production method of a catalyst layer.
2. Description of the Related Art
Hitherto, there has been made such an attempt to increase the surface area of a catalyst so as to enhance the utilization efficiency of the catalyst by providing fine particles of the catalyst and three-dimensionally dispersing the fine particles by having them carried by carbon particles or the like. Meanwhile, there have been also made such attempts to improve substance transport in a catalyst layer by forming the catalyst layer into an extremely thin layer such as of about several μm, and to increase the effective area of the catalyst by concentrating the catalyst layer in the vicinity of an electrolyte membrane. In particular, in a case of mounting a fuel cell to a small-sized electrical equipment, size reduction of the fuel cell itself is also required. For that reason, a system (air breathing system) of supplying air from air holes to an air electrode by natural diffusion without using a pump or a blower has been employed in many cases. In such air supply system, there are many cases where substance transport at an air electrode becomes a reaction rate-determining factor, and it is therefore considered that reducing the thickness of the catalyst layer is effective means for improving the reaction rate.
To deal with this, as a thin film formation means, a vapor deposition method such as sputtering is effectively used. In Japanese Patent Application Laid-Open No. 2006-049278, there is disclosed a production method of a thin film catalyst layer having a large specific surface area and high activity using a sputtering process.
Besides, there is generally known that it is preferred that in order to increase the utilization efficiency of a catalyst, the catalyst layer has an appropriate amount of fine pores or cracks. Japanese Patent Application Laid-Open No. 2002-270187 discloses a method of providing cracks in a catalyst layer by controlling drying conditions of a catalyst slurry and controlling the cracks.
However, in the catalyst layer produced by the method described in Japanese Patent Application Laid-Open No. 2006-049278, there is formed an extremely dense region in such a thickness as about 100 nm to 200 nm at an initial stage of the formation of the catalyst layer. For that reason, in a case of forming a membrane electrode assembly (MEA) by a transfer process, or in a case of forming a catalyst layer directly on a gas diffusion layer, since the dense region is formed on the gas diffusion layer side, it has been required to improve gas diffusion ability and water discharge ability, to thereby enhance the utilization efficiency of the catalyst.
Further, the method of forming fine pores or cracks in the catalyst layer described in Japanese Patent Application Laid-Open No. 2002-270187 includes making a powder catalyst into a slurry and producing cracks in a drying process of the slurry. For that reason, the method can not be applied to the catalyst layer formed by the dry method of a vapor phase as described in Japanese Patent Application Laid-Open No. 2006-049278. Therefore, there has been not known a method of forming cracks in the catalyst layer such as described in Japanese Patent Application Laid-Open No. 2006-049278.